The present invention relates to an active material for constituting a nickel electrode for use in an alkaline storage battery such as nickel-metal hydride storage battery, nickel-cadmium storage battery and so on, and a nickel positive electrode using the same.
With the current rapid and wide spread of information equipment such as portable phone, PHS, notebook-type personal computer, etc., there is a serious demand for a secondary battery that has a high energy density and exhibits excellent performance as a battery even at high ambient temperatures. There has been a demand for the development of a novel secondary battery with a high energy density as a power source for electric vehicles, as well as a battery which is suited for use in a wide range of ambient temperatures. In order to answer such demand, a provision of a high capacity to the nickel-cadmium storage battery using a conventional sintered nickel positive electrode has been realized in the field of nickel-cadmium storage battery, and a nickel-cadmium storage battery having a high energy density including a foamed metal nickel positive electrode which has a 30 to 60% higher capacity than the former electrode has been developed. Furthermore, a nickel-metal hydride battery having a higher capacity than the nickel-cadmium storage battery which includes a hydrogen storage alloy as the negative electrode has also been developed. This nickel-metal hydride storage battery has a 2-fold or higher capacity than the nickel-cadmium storage battery using the sintered nickel positive electrode.
The above-noted various high capacity alkaline storage batteries include a sintered porous nickel substrate, a three-dimensional foamed porous nickel substrate of high porosity (90% or more) or a porous nickel fiber substrate being filled with a nickel hydroxide powder as an active material at a high density. The use of such porous substrates of high porosity has led to improvements of the energy density: Compared to 400 to 500 mAh/cm3 of the conventional sintered nickel positive electrode, the recent sintered nickel positive electrode affords 450 to 500 mAh/cm3 and the foamed metal nickel positive electrode affords 550 to 650 mAh/cm3.
However, the above-mentioned nickel positive electrodes have a common drawback: the energy density is high around room temperature but low at high ambient temperatures. This may be because when charged at high ambient temperature, these electrodes are liable to evolve oxygen upon charge of nickel hydroxide to nickel oxyhydroxide. In other words, oxygen evolution at the positive electrode inhibits sufficient charge of nickel hydroxide to nickel oxyhydroxide during charge, leading to poor utilization of the active material nickel hydroxide.
The following are proposed methods for solving the above-mentioned problem.
(1) Add a cadmium oxide powder or a cadmium hydroxide powder to the positive electrode;
(2) Incorporate a cadmium oxide in a nickel hydroxide powder (see Japanese Laid-Open Patent Publication No. Sho 61-104565); and
(3) Incorporate a compound of yttrium, indium, antimony, barium or beryllium in the positive electrode (see Japanese Laid-Open Patent Publication No. Hei 4-248973).
The methods (1) and (2) are intended to improve the utilization of nickel hydroxide active material at high ambient temperature by the presence of a cadmium oxide inside or in close contact with a nickel hydroxide powder. These methods can best utilize 80% or so of the nickel hydroxide active material at high ambient temperature. In order to further increase the utilization of nickel hydroxide at high ambient temperature, there is a need to increase the content of cadmium oxide in the nickel hydroxide or in the nickel positive electrode. However, there is a problem that increased content of cadmium oxide improves the utilization of nickel hydroxide at high ambient temperature to as high as 90% or so, but adversely reduces its utilization around room temperature.
From the aspect of current issue of environmental pollution, nickel-metal hydride storage battery which is free of heavy metal cadmium has been noted recently. Therefore, the use of nickel positive electrode containing a cadmium oxide is not suited for nickel-metal hydride storage battery.
The last method (3) adsorbs a compound of yttrium, indium, antimony, etc. on the surface of nickel oxide active material, expecting the following effects: (i) an elevation in oxygen evolution overvoltage as a competitive reaction in response to charge at high ambient temperatures, (ii) an increase in charge efficiency, that is, oxidation of nickel hydroxide to nickel oxyhydroxide, and (iii) an improvement of utilization at high ambient temperatures. However, simple application of this method only does not offer those expected effects due to non-homogeneous distribution of the additive in the active material paste or others. In order to have prominent effects, the use of additive in a large amount becomes mandatory, but this hinders realization of a high capacity battery.
The object of the present invention is to provide a nickel hydroxide active material for use in nickel positive electrode that can solve the above-mentioned problems and offer an alkaline storage battery having a higher capacity and a longer cycle life.
The present invention is based on the discovery from an experiment focusing on the fact that the amount of impurities contained in a nickel hydroxide active material powder, particularly the impurity of sulfate ion (SO42xe2x88x92) present in the nickel hydroxide powder obtained from nickel sulfate plays a significant role in determining electrode characteristics, and that regulation of the amount of sulfate ion as an impurity can improve the charge efficiency of nickel hydroxide active material at high temperatures and is effective for elongating the cycle life of the resultant battery.
The present invention provides an active material for constituting a nickel electrode comprising a nickel hydroxide powder formed from nickel sulfate wherein the content of impurity SO42xe2x88x92 in the crystal of nickel hydroxide is 0.4 wt % or less (hereinafter wt % is represented by % simply).
The nickel hydroxide used here is preferably solid solution nickel hydroxide incorporating therein at least one element selected from the group consisting of cobalt, cadmium, zinc and magnesium.
The present invention can improve the utilization of positive electrode active material at high ambient temperatures and increase the filling amount of the nickel hydroxide by reducing the conventional amount of additive. The present invention therefore can provide a high performance alkaline storage battery operable in a wide range of ambient temperature.